1. Field of the Invention
This invention pertains to marine seismic surveying. More particularly, method is provided for determining orientation of horizontal receivers used for detecting shear waves in ocean-bottom cable surveys.
2. Description of the Related Art
Marine seismic surveys are often obtained using hydrophones connected to electrical cables that are contained in streamers towed behind boats. Hydrophones measure pressure waves (P-waves) that are reflected and refracted through the water and earth layers. There is no need to know the orientation of hydrophones. Seismic data acquisition in shallow water was carried out many years ago using geophones connected to stationary cables that were placed on the bottom by hand. Soon there was a need to extend operations to deeper water and ocean-bottom cable (OBC) came into use, which required the use of small boats for cable layout. As long as hydrophones and single component vertical geophones were used there was a method to insure orientation of the geophones in the desired vertical direction. In recent years, there has been renewed interest in using stationary receiver spreads on the seafloor in deeper water with three-component (3C) geophones. Often a hydrophone is also used in proximity to a 3C geophone to produce a four-component (4C) receiver. This interest in 4C OBC systems has been the result of several factors, including the development of techniques for suppressing water-column reverberation noise ("Attenuation of Water-Column Reverberations Using Pressure and Velocity Detectors in a Water-Bottom Cable," 59th Ann. Internat. Mtg., Soc. Expl. Geophys., Dallas (1989), and the demonstration that mode-converted shear waves can be used to great advantage to obtain additional information about the subsurface. Multicomponent data that describe the total vector wavefield of P-wave and S-wave particle motion are necessary to maximize the information that can be gleaned from a seismic survey.
Location of ocean-bottom receivers must be known for data acquisition. This can be provided by, for example, the method described by Edington, et al ("Ocean Bottom Receiver Location," 25th Ann. OTC, Houston, May 3-6, 1993). But, for three-component geophones, orientation must also be known. The geophones are normally constructed with the vertical geophone gimballed to remain vertical regardless of orientation of the geophone case. The important orientation problem remaining for OBC systems is then measuring the orientation or azimuth of the horizontal receivers. These receivers record particle motion of the S-waves, and the particle motion can range over all azimuths, particularly for a 3-D survey where a single receiver station will be surrounded by sources. When the horizontal orientation is not known, S-wave data detected by the horizontal receivers cannot be combined properly for vector wavefield processing.
The orientation of three-component geophones in an ocean-bottom cable survey can differ from one station to the next. This is because as the cable attached to the geophones and hydrophones is released from the boat, it does not always reach the sea floor in a straight configuration. The cable and receivers may reach the seafloor with curves in the cable. Curvature in the cable and, in some cases, uneven bottom surfaces, can cause horizontal receivers to be out of alignment with their intended direction. Lack of alignment may occur for all acquisition methodologies to some degree, whether cables are dragged or draped or for units planted on the sea floor by remotely operated vehicles.
In the art of recording vertical seismic profiles around wells, there is also a need for orientation of the horizontal components at each depth in a borehole. A solution of DiSiena et al. (Vertical Seismic Profiling, Geophysical Press, 1984, pages 177-188) analyzed the amplitude distribution of the direct downgoing P-wave arrival detected by the x and y horizontal receivers. A histogram was constructed of the magnitude versus the instantaneous orientation for all the time samples in a window including the direct arrival. The bin with the maximum peak in the histogram was interpreted as the horizontal direction aligned with the P-wave particle motion and thus the source-receiver direction. Since the histogram analysis referenced an angle between 0.degree. and 180.degree., the vertical component was used to remove horizontal polarity ambiguities.
What is needed is a method for determining the orientations of the horizontal receivers during conventional OBC surveys without the need for additional equipment. This method should provide the accuracy in orientation that provides for combining the various horizontal receivers properly for vector wave field processing of the seismic data collected.